Heading indicator and method of using

ABSTRACT

A heading indicator is disclosed which utilizes a one degree of freedom platform stabilized by a two degree of freedom dry flexure gyro. The output of one of the sensitive axis is coupled through an amplifier to the corresponding torquer in the gyro and selectively through an another amplifier to the opposite torquer. The other sensitive axis output is coupled through an amplifier to the platform. The indicator initializes at true north and is then switched to a directional gyro mode.

BACKGROUND OF THE INVENTION

This invention relates to heading instruments in general and moreparticularly to a low cost instrument containing a visual display ofheading for tanks.

The present low cost technique for providing heading information intanks requires a magnetic sensor to initialize the system and adirectional gyroscope for maintaining the alignment. In a system of thisnature the visual read out of heading information is an indicator cardmounted on the control panel which is servoed to a synchro on thedirectional gyroscope.

The primary disadvantage of this system is that it uses a magneticsensor for alignment which can be affected by large iron or steelstructures. In tanks for example, a turret would have to be in a fixedposition during alignment in order to avoid degrading the accuracy ofthe system. Also the proximity of other armored vehicles at this timecan introduce errors in the heading.

Another disadvantage of this system is that the components take up aconsiderable amount of space, something that is at a premium in tanks.

Some expensive devices have utilized a gyrocompassing function with theuse of accelerometers and bubbles which in addition of increasing thecost have increased the overall complexity of the system.

It is an object of this invention to provide a low cost headingindicator which is insensitive to weak magnetic fields of ferrousstructures proximate to the device.

It is another object of the invention to provide a low cost headingindicator that is smaller in volume than the presently used device.

Another object of the invention is to combine the functions of agyrocompass and a directional gyro in a single device without the needfor accelerometers or bubbles.

Another object of this invention is to provide a system that alignsitself to true North within 5 minutes, that is insensitive to magneticanomalies and that can then be switched to a directional gyroscope modeto give heading indications.

It is another object of the invention to provide a low cost headingindicator with a direct gyro readout providing heading information.

SUMMARY OF THE INVENTION

These objects are attained in a low cost heading indicator whichincludes a single degree of freedom platform stabilized by a two degreeof freedom gyroscope. A circuit is provided for initially utilizing thegyroscope in a gyrocompass mode thereby aligning the gyroscope to trueNorth and then selectively utilizing the gyroscope as a directionalgyroscope. This result is obtained by coupling one of the sensivitiveaxes of the gyroscope through an amplifier to the corresponding torquer,and selectively through an amplifier to the opposite torquer. The othersensitive axis of the gyro is coupled through an amplifier to theplatform torquer. A 45 degree conically shaped compass card is mountedon top of the case of the gyroscope to provide direct gyro readout ofheading information. The device uses the gyro rate capture current as adirect measure from North to drive the gyrocompass amplifier.

The advantages of the proposed system include its small size and theavoidance of magnetic devices or bubble sensor. Additionally, it isunnecessary to provide a servo mechanism to couple the gyroscope outputto the readout device. The truncated conical compass card provides aheading display visable from either a horizontal or vertical position.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective schematic view of the low cost heading indicatorsystem of the present invention.

FIG. 2 is a functional block diagram of the system.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a perspective schematic view of indicator to be used in atank. The indicator comprises a base 6 which supports a single degree offreedom platform 10, which is rotatable about a vertical axis (azimuth).The platform 10 is stabilized by a two degree of freedom dry flexuresuspended gyroscope 11. The gyroscope 11 is oriented so that its twosensitive axes lie along the azimuth axis 13 and an east axis 12. Thegyroscope 11 is mounted on the platform 10 and includes a case attachedto an azimuth gimbal 14. To indicate heading, the system includes atruncated 45 deg. conically shaped compass card 16 which is mounteddirectly on azimuth gimbal 14. The invention overcomes the need of servomechanisms by directly mounting the compass card on the gimbal. Theconical shape of the compass card provides a visual display of headingwhen viewed from either the side or the top of the unit.

The gyroscope 11 includes a flywheel, the case 15, and a motor. The case15 is mounted on the platform 10 and the flywheel is suspended withinthe case suspension. The motor maintains the flywheel rotating at apredetermined rate.

The gyroscope 11 also includes an east pickoff 19 (see FIG. 2) forsensing the relative rotation of the case about the east axis, an eastaxis torquer 20 for restoring movement about the east axis 12, anazimuth axis pickoff 21 to sense the relative rotation about the azimuthaxis 13, and an azimuth axis torquer 22 for movement about the azimuthaxis.

An azimuth gimbal torquer 23 is provided to rotate platform 10 about theazimuth axis.

As shown in FIG. 2 the east axis pickoff 19 is coupled to the east axistorquer through a rate capture amplifier 24. The output of the ratecapture amplifier 24 is provided to the azimuth axis torquer 22 througha gyrocompass amplifier 25. A mode relay 26 is disposed between thegyrocompass amplifier 25 and the azimuth axis torquer 22.

The azimuth axis pickoff 21 is coupled to the azimuth gimbal torquer 23by means of an azimuth isolation amplifier 27 which causes the gimbaltorquer to displace the gimbal to keep it aligned with the flywheel.

A clock and countdown circuitry 30 is connected to a pickoff excitationsupply 31. The clock and countdown circuitry 30 includes a crystaloscillator clock which produces a DTL logic level square wave, andcountdown logic which utilizes the clock input to produce a square wavefor the input to the pick off excitation supply 31. The pickoffexcitation supply 31 is an amplifier which utilizes the logic input toproduce a stable sine wave with low distortion. This is achieved bycombination of filtering and feedback. The sine wave excitation signalgenerated by the pick off excitation supply 31 is used to excite theeast axis pickoff 19 and the azimuth axis pick off 21. The signalgenerated by the azimuth axis pickoff 21 is amplified by azimuthisolation amplifier 27, whose output drives the azimuth torquer 23. Theazimuth isolation amplifier 27 contains a proportional gain channel anda parallel integral channel to achieve overall servo loop compensationwith adequate gain and phase margin. The two signals are summed in anoperational amplifier whose output drives an H bridge output circuitwith the azimuth gimbal torquer 23 as the load, and a relay in serieswith the load is used to initially close the loop and also to open theloop when a BITE circuit 36 indicates a fault.

The BITE circuit 36 is a built in test equipment which basically checksthe gimbal loop, which includes azimuth axis pickoff 21, azimuthisolation amplifier 27 and azimuth gimbal torquer 23; and thegyrocompass loop, which includes the east axis pick off 19, the eastrate capture amplifier 24 the gyro compass amplifier 25 and the azimuthaxis torquer 22. Once the initial system transients have been settled,the BITE circuits 36 are enabled. Normally both the gimbal loop and thegyrocompass loop are always at null, but if a large signal(approximately 10 volts DC) appears at either of these loop outputs foran extended time period (250 ms) the BITE circuit 36 issues a faultwarning. In the case of a gimbal loop fault, the relay in series withthe gimbal torquer is open.

The signal generated by the east axis pickoff is fed into the east ratecapture amplifier 24 which in conventional fashion includes an ACpre-amplifier and a synchronous demodulator, coupled to the preamplifierto achieve quadrature rejection. Also included are operationalamplifiers for integration and loop servo compensation. The next stagehas a notch filter that uses an operational amplifier withresistor/capacitors in a parallel tee configuration to achieve highattenuation. The output stage of this loop is arranged as a currentamplifier with feedback from a resistor in series with the east axistorquer 20. The loop gain is changed by shunting a load resistor inseries with the east axis torquer 20.

The output from the east rate capture amplifier 24 is fed into agyrocompass amplifier 25 through the mode relay 26. The gyrocompassamplifier feeds a power summing amplifier in a current feedbackconfiguration in the gyrocompass mode. In the directional gyro mode themain input of the summing amplifiers is grounded while a correctionsignal of an azimuth gyro drift compensation may be fed into the summingamplifier input. Low drift operational amplifiers are utilized in thisconfiguration to achieve the desired system accuracy. The output of thegyrocompass amplifier 25 is fed into the azimuth axis torquer 22 whichis utilized to drive the flywheel about the azimuth axis. The output ofthe gyrocompass amplifier 25 is also coupled to BITE circuit 36 withwhich checks the signal in the same way as it checked the output of theazimuth isolation amplifier 27 above.

The clock and countdown circuitry 30 is connected to a gyro wheel supply45 which counts down the logic signal from the clock and countdowncircuitry 30 to produce an input to a logic countdown, that in turnproduces 4 phases for the input signal to the gyro wheel supply. Thegryro wheel supply 45 contains two transition H bridges that are drivenby the gyro wheel supply input logic signals. The gyro wheel supply iscoupled to the gyro motor. Each phase winding of the gyro motor is theload for the pair of H bridges. The excitation for the bridges is asingle ended DC voltage. The input logic has frequency detectioncircuitry to detect loss of signal and prevent the possibility ofdamaging the motor.

The clock and countdown circuitry 30 provides a slip sync signal ofwhich is used as the input signal to a sequencer 50. The sequencerutilizes the logic frequency to develop the following approximate timesfrom additional countdown logic circuitry and relay drivers. T0=0, T1=30seconds, T2=31 seconds, T3=300 seconds. At T0 the switch 52 is closed,activating the clock and countdown circuitry 30 and also energizing thegyroscope 11. The T1 timing signal turns on a capture loop relay 51 thatcloses the east rate capture loop, which includes the east pick off 19,the east rate capture amplifier 24 and the each torquer 20; and thegimbal isolation loop described above. At T2 the gyrocompass loop isclosed by energizing the mode relay 26. At T3 the mode relay 26 isdeenergized to open the gyrocompass loop and the BITE circuit isenabled. Any fault condition that exists after the BITE is enabled willkeep the malfunction lamp illuminated. A power supply 53 utilizes 28volt DC battery voltage as an input and utilizes pulse width modulationto achieve regulation and efficiency.

The operation of the system described above can be better understood byan explanation of some general principles of gyroscope physics and anexample of the operation of the different modes.

At any local latitude, λ, the earth's rotational velocity, W_(e), can beresolved into two components, a horizontal component (W_(H)) and avertical component (W_(V)). The horizontal component of earth's ratelies in a plane which is perpendicular to local gravity. The verticalcomponent of earth's rate lies in the same vertical plane as localgravity. This vertical plane is aligned in a north-south direction.

Consider the platform 10 with the azimuth gimbal 14 approximatelyvertical (FIG. 1). The two degrees of freedom gyro 11 is mounted on thisgimbal 14. One axis of the gyro, the east axis, 12 is captured back onto itself through the east rate capture amplifier 24. The other axis,the azimuth axis 13, is captured to the platform 10 through the azimuthisolation amplifier 27.

The east rate capture amplifier 24 captures the flywheel to the caseabout the east axis 12. The azimuth isolation amplifier 27 captures thecase to the flywheel about the azimuth axis 13.

Assuming that the base of the Heading Indicator is in a horizontal planewith the east axis at an initial value ψ_(IN) from the East-Westreference and the azimuth axis is vertical, then the heading referencesystem is not yet useful in providing a north reference since the eastgyro axis is not aligned to its reference, namely East. The Northreference is established through a gyrocompassing process which isdefined to be complete when the east axis is orthogonal to earth's rate.This occurs when ψ is zero; that is when the east axis is East.

For the purposes of following the gyrocompasing process assume that theazimuth gimbal is initially positioned such that the east axis is ψ_(IN)degrees from East, ψ≠0. In this position the east axis pickoff willsense a component of horizontal earth's rate.

    (W.sub.H)(Sin ψ.sub.IN)

The gyro case will rotate about the east axis of the flywheel at thisrate. The east rate capture amplifier 24 provides a signal to the eastaxis torquer 20 to move the flywheel so that it follows the case 15 andmaintains the east axis pickoff 19 at zero.

A measure of the east gyro torquer signal is amplified and sent to thegyrocompass amplifier 25. The gyrocompass amplifier output signal isapplied to the azimuth axis torquer 22 which displaces the flywheel withrespect to the case about the azimuth axis 13. The azimuth axis pickoff21 provides an electrical signal indicating a relative motion betweenthe flywheel and the gimbal 14. The signal is processed through theazimuth isolation amplifier 27 and drives the azimuth gimbal torquer 23so that the gimbal 14 follows the flywheel and maintains the azimuthaxis pickoff 21 at zero. The direction that the azimuth gimbal rotatesis such to reduce the value of ψ.

This process continues until east gyro pickoff 17 senses no component ofhorizontal earth's rate; that is ψ=0 which usually takes about 5 min.The system has then finished gyrocompassing.

After gyrocompassing is finished the system switches to the directionalgyro (DG) mode. This is done by opening the loop between the east ratecapture amplifier 24 and the azimuth axis torquer 22 with mode relay 26,and can be accomplished manually or with a sequencer.

The purpose of the DG mode is to maintain the alignment that wasachieved during gyrocompassing while the vehicle is travelling. This isaccomplished through the east rate capture amplifier 24 and the azimuthisolation amplifier 27 electronics previously described. The east ratecapture amplifier 24 keeps the flywheel from being disturbed, bytorquing the flywheel so that it can follow the gimbal under all typesof base motion that the vehicle will impose on the system. The azimuthaxis 13 maintains the North alignment. Any vehicle motion about azimuththat may disturb the gimbal is sensed by the azimuth axis pickoff 21.The aximuth axis pickoff 21 provides a signal to the azimuth gimbaltorquer 23 which keeps the gimbal 14 captured to flywheel which is fixedin space providing the North reference.

What is claimed is:
 1. A method of indicating heading comprising thesteps of:disposing a single degree of freedom platform on a base so asto be rotatable about a vertical axis; suspending a two degree offreedom gyro, having a first and second sensitive axes, on said platformsuch that said second sensitive axis is aligned with said vertical axis;sensing rotation about said first sensitive axis; developing a firstoutput proportional to the rotation about said first sensitive axis;sensing rotation about said second sensitive axis; developing a secondoutput proportional to the rotation about said second sensitive axis;torquing said gyro about said first sensitive axis an amountproportional to said first output; torquing said gyro about said secondsensitive axis an amount proportional to said first output for apredetermined amount of time; and rotating said platform an amountproportional to the second output.